The decision to attempt a femorodistal bypass graft is usually based on the pre-operative demonstration of an adequate calf vessel runoff. It is an advantage to know the state of the runoff vessels as this may affect the level of the distal anastomosis and influence graft patency. When the popliteal artery is occluded it is essential to detect which calf vessel, if any, is patent as this will determine whether grafting is at all possible. Assessment of the pedal arch and its calf vessel connections may also influence the site of the distal anastomosis.
The recent upsurge in popularity of femorodistal grafts for limb salvage has highlighted the shortcomings of conventional pre-operative arteriography. About one-quarter of crural or pedal arteries that are judged patent on Doppler ultrasonography or direct operative exploration may fail to opacify. The use of specialized techniques such as reactive hyperaemia, vasodilators, intra-operative arteriography, or digital subtraction arteriography may increase the visualization of distal vessels but they are not always practicable or available.
Conventional pre-operative arteriography may fail to demonstrate patent calf and foot vessels, especially in the presence of severe ischaemia. Although distal vessels should be judged occluded only if there is filling of the small collaterals, this may not be achieved on the initial arteriograms. It may be uncertain whether failure to demonstrate distal vessels is due to technical factors or due to occlusion. Repeat arteriograms incur additional expense and delay with no guarantee of improved definition. Worse still, it may be assumed that vessels are occluded simply because they have not been filled with contrast medium, and the patient may thus be denied the chance of reconstruction.
Doppler ultrasonography may detect patent crural or pedal vessels that are missed on arteriography but may itself miss patent vessels because of severely damped signals due to a low perfusion pressure. Diligent searching over the area of the vessel without pressure from the probe on the skin and increasing the perfusion pressure by making the feet dependent may help, but require patience and expertise.
An ideal system for assessing distal vessel patency would be non-invasive, safe, simple and rapid to perform. Standard Doppler ultrasound is useful but may also miss patent vessels if signals are severely damped despite a meticulous technique.